Topical drug delivery, compared to conventional routes of administration such as oral or parenteral delivery, is both potentially advantageous since it avoids active principle degradation in gastrointestinal tract and first pass hepatic metabolism and it is more acceptable by patients.
However the skin, which consists of two layers, the deeper one or dermis and the external layer or epidermis, behaves as a difficult to permeate barrier for most drug substances.
The deeper layer or dermis, whose thickness is between 0.3 and 4 mm, consists of connective tissue embedded with blood vessels, pilo-sebaceous units (hair follicles and sebaceous glands) as well as nerve endings which makes skin a true sense organ. At dermis level, active principles can cross the capillary walls to enter into the circulatory system and reach different tissues. The outermost layer of the skin or epidermis, whose thickness is between 50 and 150 μm, is covered by a hydrolipidic film and performs a barrier function against microorganisms and other exogenous molecules from the surrounding environment. Keratinocytes are the typical epidermis cells that originate at the innermost layer close to dermis and undergo a gradual differentiation process called keratinization ending with migration to surface to form a horny layer of dead cells (statum corneum) with thickness between 10 and 30 μm.
The horny layer acts as an effective barrier limiting the passage of active principles whose rate of transdermal absorption correlates with the generally very low rate of their penetration through the horny layer. Due to this barrier effect, the topical administration of drugs normally results in a reduced bioavailability.
Different approaches have been investigated to improve the diffusion of drugs through the skin including physicochemical methods based on the use of penetration enhancers such as dimethylsulphoxide, fatty acids, propylene glycol and urea [Williams A C e Barry B W, 1992] as well physical methods including, among others, iontophoresis, electroporation and low-frequency ultrasound [Lavon I e Kost J, 2004] or a combined application of both physical methods and chemical enhancers.
A different approach to improve the transdermal diffusion of drugs is based on the carrier properties of vesicular structures generally indicated as liposomes. Liposomes are microscopic vesicles of 50-500 nm containing amphiphatic phospholipids which are molecules that have a polar head and a lipidic tail. In aqueous environment the hydrophilic heads line up to form a surface facing the water while the hydrophobic tails, repelled by water, line up to form a surface away from the water; moreover, the hydrophobic tails of one layer interact with the hydrophobic tails of another layer to form closed bilayer structures organized as uni-lamellar or multi-lamellar vesicles containing one or more aqueous compartments.
The chemicophysical properties of liposomes can be easily modified by variations of preparation methods and/or by including in their composition different substances such as, for example, cholesterol which increases the stability of lipidic bilayer, or acidic or basic lipid molecules which modify the electrical charge of vesicular surfaces and reduce liposome aggregation.
Hydrophilic compounds are solubilized in liposomes aqueous compartments and hydrophobic compounds enter into the lipidic bilayers while amphipathic molecules are distributed with their polar portions in water and apolar portions in lipidic lamellae
Moreover, ingredients used to prepare liposomes are safe, non allergenic and being fully compatible with biological membranes enable the interaction of liposomes with cells.
Different mechanisms have been suggested to explain the interaction between liposomes and cells and some of them can possibly occur together. The most plausible mechanism is endocytosis where intact liposomes are phagocytated by cells and internalized into lysosomes where the drugs are released by phospholipase degradation of lipidic bilayers. Other putative mechanisms are based on lipidic substance exchange due to fusion of liposome bilayers with cell membranes followed by distribution of vesicle content between cytoplasmic compartment and cell membranes in dependence of its physicochemical properties.
The use of liposomes for targeted drug delivery at dermis and epidermis level with the aim to reduce systemic absorption and undesired effects has been first suggested in 1980 [Mezei M e Gulasekharam V, 1980; Mezei M, 1988] and then studied by many authors [Lichtenberg D e Barenholz Y, 1988; Woodle M C e Papandjopoulos D, 1989, Sinico C et. al 2005, Manconi M et. Al 2006]. In some cases, liposomes can be used for transdermal permeation of drugs or cosmetic agents through skin appendages, such as hair follicles and sebaceous glands since, at this level, in dependence of liposome composition and preparation technique, active principles can cross the capillary walls and reach different tissues and organs from blood circulation [El Maghraby G. M et al., 2006]. In most cases, however, the use of liposomes as carriers for topical and/or transdermal administration of active principles is of little or no value since conventional liposomes do not penetrate the skin but rather remain confined to the upper layers of the stratum corneum. Later on, new classes of lipidic vesicles have been developed, namely deformable lipid vesicles and ethosomes, where the presence of specific additives modify the chemicophysical and functional properties of conventional liposomes enabling a more efficient delivery of drugs to deeper layers of the skin.
Deformable or flexible liposomes (also called Transferomes®) incorporate, besides the basal phospholipidic component, single chain surfactants with a high radius of curvature (selected, for example, among sodium cholate, sodium deoxycholate, potassium glycyrrhizinate, Span® 60 sorbitane monostearate, Span® 65 sorbinate tristearate, Span® 80 sorbitane monooleate, Tween® 20 polyethylene glycol sorbitan monolaureate, Tween® 60 polyethylene glycol sorbitan monostearate, Tween® 80 polyethylene glycol sorbitan monooleate) which are able to destabilizes vesicle lipidic bilayers and increase liposome deformability given them a higher skin penetration capability [Cevc G, Blume G, 1992].
Transferosomes are morphologically similar to standard liposomes even if they are functionally different being sufficiently deformable and flexible to penetrate pores much smaller than their own size. The mechanism of action of transferosomes is based both on a better capability of intact deformable vesicles to enter the stratum corneum and on subsequent modification and destabilization of intercellular lipidic structures which facilitate the diffusion of free drug molecules from the vesicles.
However, since the transport mechanisms of these deformable vesicles could partially depend on the physicochemical properties of incorporated drugs, the preparation of transferosomes need to be optimized on a case-by-case basis [Elsayed M M A et al., 2006].
A different approach to increase the fluidity of lipidic membranes of liposomes in order to improve their function as carriers of drugs and/or cosmetic products is represented by lipid vesicles called ethosomes composed of phospholipids, water and high concentrations, generally between 20% and 50%, of ethanol [Touitou E, U.S. Pat. No. 5,716,638]. High ethanol concentrations reduce the ethosomes size and, in general, increase their encapsulation efficiency for a wide range of active principles including lipophilic molecules resulting in an effective delivery system for topical administration of drugs [Touitou E at al., 2000].
The action of ethosomes is possibly due to a synergistic mechanism between ethanol, lipid vesicles and skin lipids where the presence of ethanol could provide the vesicles with flexible characteristics for a better penetration into deeper layers of the skin as well as display its well-known permeation enhancing effect [Williams A, 2003].
Besides, ethosomes penetrated into the deep layers of the skin could fuse with skin lipids promoting the release of drugs at dermis level and possibly their transdermal absorption.
However, because of the high content of ethanol, the use of ethosomes as delivery system for drugs and/or cosmetic products may have unfavourable or irritanting effects in case of application on wounded or otherwise injured skin.